1. Field of the Invention
The present invention relates to oscillators and semiconductor devices, and particularly to an oscillator and a semiconductor device which output four-phase signals.
2. Description of the Related Art
Conventionally, there have been oscillators which output multiple-phase signals (the oscillators are disclosed, for example, in paragraph numbers 0015 to 0026 and FIG. 1 in Japanese Unexamined Patent Publication No.6-152335 (1994), and paragraph numbers 0027 to 0029 and FIG. 1 in Japanese Unexamined Patent Publication No. 6-188634 (1994)). There have also been voltage-controlled oscillators (LC-VCO) which are formed of two LC resonant circuits and an inverter ring which couples the resonant circuits, and which output four-phase signals (the oscillators are disclosed, for example, in Behzad Razavi, “Design of Integrated Circuits for Optical Communications,” first edition, United States of America, McGraw-Hill, Sep. 12, 2002, pp. 235-241, and Marc Tiebout, “Low-Power Low-Phase-Noise Differentially Tuned Quadrature VCO Design in Standard CMOS,” IEEE Journal of Solid-State Circuits, Vol. 36, No. 7, July, 2001, pp. 1018-1024).
FIG. 13 is a schematic diagram of a conventional oscillator formed of LC resonant circuits and an inverter ring.
As shown in FIG. 13, the oscillator has LC resonant circuits 100 and 110, and a coupling circuit 120.
The LC resonant circuits 100 and 110 each output clock signals having a phase difference of 180 degrees. The coupling circuit 120 couples the clock signals output from the LC resonant circuits 100 and 110 so as to have phase relationships of 0 degrees, 90 degrees, 180 degrees, and 270 degrees.
The LC resonant circuit 100 includes inverters Z101 and Z102, an inductor L101, and a variable capacitor VC101. The inverters Z101 and Z102 are turned on/off at the resonant frequency determined by the inductor L101 and the variable capacitor VC101, and output clock signals having a phase difference of 180 degrees to the coupling circuit 120.
The LC resonant circuit 110 includes inverters Z111 and Z112, an inductor L111, and a variable capacitor VC111. The inverters Z111 and Z112, the inductor L111, and the variable capacitor VC111 have the same parameters as the inverters Z101 and Z102, the inductor L101, and the variable capacitor VC101 in the LC resonant circuit 100. The LC resonant circuit 110 operates in the same way as the LC resonant circuit 100, and outputs clock signals having a phase difference of 180 degrees to the coupling circuit 120.
The variable capacitors VC101 and VC111 change their capacitances according to applied voltages. When the voltages applied to the variable capacitors VC101 and VC111 are controlled, the frequencies of the clock signals output from the LC resonant circuits 100 and 110 are controlled accordingly.
The coupling circuit 120 has inverters Z121 to Z124. The inverters Z121 to Z124 are connected in a ring manner to form an inverter ring. The coupling circuit 120 couples the LC resonant circuit 100 and the LC resonant circuit 110 so as to have a phase difference of 90 degrees between the LC resonant circuits 100 and 110.
When the inverters Z101 and Z102 of the LC resonant circuit 100 output clock signals having phases of 0 degrees and 180 degrees, for example, the inverters Z111 and Z112 of the LC resonant circuit 110 output, for example, clock signals having phases of 90 degrees and 270 degrees, which gain by 90 degrees (0+90=90 and 180+90=270) from the outputs of the LC resonant circuit 100. Alternatively, the inverters Z112 and Z111 output clock signals having phases of 270 degrees and 90 degrees, which lose by 90 degrees (0−90=270 and 180−90=90) from the outputs of the LC resonant circuit 100. From the oscillator, the four-phase clock signals having phases of 0 degrees, 90 degrees, 180 degrees, and 270 degrees are obtained.
If the coupling circuit 120 has a high gain, the oscillator does not oscillate. Therefore, the LC resonant circuits 100 and 110 and the coupling circuit 120 need to have appropriate gains.
The conventional oscillator ensures that the LC resonant circuits 100 and 110 output clock signals having phases of 0 degrees and 180 degrees.
However, it is not ensured by the coupling circuit 120 that the phases of the clock signals output from the LC resonant circuit 110 gain or lose by 90 degrees with respect to the phases of the clock signals output from the LC resonant circuit 100 (it is not ensured that the phase of the output of the inverter Z121 gains by 90 degrees or loses by 90 degrees (gains by 270 degrees) with respect to the phase of the input of the inverter Z121.)